This invention relates in general to power control for mobile communication devices, and more particularly to optimizing the efficiency of power amplifiers in mobile communication devices.
Portable radio transceivers, such as those found in ordinary communication devices, contain both a radio frequency (RF) amplifier and an audio amplifier. The RF amplifier amplifies the modulated radio signal that is transmitted by the device, while the audio amplifier amplifies the audio signal received by the device. In all portable or mobile communication devices battery life is an important design consideration. In most mobile communication devices the RF amplifier requires a relatively high amount of power. Accordingly, they are usually designed to be as efficient as permissible within other design criteria so as to reduce the power demand, and thus increase battery life. In many mobile communication devices, such as cellular phones, the audio amplifier does not use as much power because driving an earpiece speaker, even at full volume, doesn""t require nearly as much power as the RF amplifier. However, in some mobile communication devices, such as trunked radios, the audio power is considerable because they drive a speaker that is meant to be heard at a distance. Furthermore, some cellular phones now provide speakerphone capability, which, liked trunked radio, is a form of half-duplex communication.
At the same time, several modulation formats have become popular, including some linear modulation types, such as, for example, quadrature amplitude modulation. Such modulation is used in several digital communication systems. However, one disadvantage of linear modulation is that the efficiency decreases as the difference between output and supply voltage increases. For linear RF amplifiers that are used with different power settings, the inefficiency may rival the power savings when transmitting at lower output power levels. However, the advantage of narrow channel spectral use is often found to be advantageous enough to accept such inefficiency, or to add circuitry to the device to counteract the inefficiency.
Just as linear RF amplifiers suffer from inefficiency when the voltage of the output drops significantly below the supply voltage, so to do audio amplifiers with variable power output. Since an audio signal has a variable amplitude, it is similar to linearly modulated RF signals. For the most part, the inefficiency of an audio amplifier is much smaller in most mobile communication devices, such as cellular telephones, because the earpiece speaker is driven with a small amount of power compared to the RFPA power. However, many trunked or two way radios use a much louder audio output, and some cellular or mobile telephones have a speakerphone mode of operation that likewise has a much higher power audio output compared to a conventional earpiece speaker output. The reason for the higher output is, obviously, so the user can hear from a distance the received audio signal.
In the design of portable and mobile battery powered devices the battery life is an important consideration. As such, designers strive to reduce the power demand and increase efficiency where possible. In a portable transceiver having both a radio frequency power amplifier and a high output audio power amplifier, the power consumption is generally higher than a conventional device having only a low power, earpiece audio output circuit. Therefore, in a transceiver device having a high output audio power amplifier, there is a need for a method and apparatus to increase the efficiency of transceiver operation to preserve battery life.